1. Field of the Invention
The present invention relates to retrieval tools. More specifically, the present invention pertains to overshot tools used for retrieving a cylindrical body that has become lodged or otherwise lost within a wellbore.
2. Description of the Related Art
In the formation of a hydrocarbon or other wellbore, a cylindrical hole is formed vertically through a series of earth formations. Typically, the wellbore is first formed by rotating a drill bit downward at the lower end of a drill string. Traditionally, the drill string has been lowered into the wellbore by threadedly connecting a series of pipe joints, and then rotating those pipe joints in order to impart rotational movement to the drill bit downhole.
During the drilling of a wellbore, it is not uncommon for the operator of the rig to lose the ability to rotate the drill bit downhole. In this respect, those of skill in the art will understand that wellbores generally are not formed in a perfectly vertical plane; instead, the movement of the drill bit tends to form a “corkscrew” profile as the drill bit moves downward into the earth. This, in turn, creates tremendous frictional forces, or “drag,” between a drill string and a surrounding earth formation. A lower portion of the drill string may become fatigued and separate due to the high tongue forces imparted during the drilling process. When this occurs, the upper section of drill pipe must be removed from the hole, and a fishing tool deployed in an attempt to retrieve the parted lower portion of drill pipe and connected drill bit.
In order to retrieve the parted drill string left downhole, an overshot tool has been traditionally used. A well-known example of such an overshot tool is the Bowen Series 150 releasing and circulation overshot tool. An exemplary overshot tool of the Bowen-type is shown at 10 in FIG. 1. This overshot tool, and others of similar arrangement, has been known and used in the oil industry for many years.
As seen in FIG. 1, the overshot tool 10 first comprises a body 12. The body 12 defines an elongated tubular member having an upper end and a lower end. The upper end and the lower end are each internally threaded. The upper end is threadedly connected to a top sub 11. The top sub 11 serves as a connector between the body 12 of the overshot tool 10 and the working string (not shown). The lower end, in turn, is threadedly connected to a guide member 14. The guide member 14 aids in running the overshot tool 10 into the wellbore and over the parted drill string downhole for retrieval. Other tools, such as a circular milling tool, may also be attached below the body of the overshot tool. The milling tool (not shown) typically defines a cylindrical body having carbide material disposed there around.
As shown in FIG. 1, the inner diameter of the overshot tool body 12 has a serrated profile. This means that a series of ramp surfaces 16 are placed along the inner diameter of the body 12. In one arrangement, the ramp surfaces 16 are spiraled along the inner surface of the body.
A separate gripping member 18 is disposed within the body 12. Where the ramp surfaces 16 are spiraled, the gripping member 18 is also spiraled. For the overshot tool 10 of FIG. 1, the spiraled gripping member 18 is configured to nest within the ramp surfaces 16 of the body 12. This means that the outer diameter of the gripping member 18 is configured to ride along the ramps 16 of the inner surface of the tool body 12. An example of such a gripping member 18 for the overshot tool 10 is shown in FIG. 2.
Referring to FIG. 2, it can be seen that the gripping member 18 has a generally smooth outer surface, but a grooved inner surface. The inner grooves define wickers 19 used for gripping the outer diameter of a parted drill string or other cylindrical body downhole (sometimes referred to as a “fish”). To accomplish the gripping function, the wickers 19 are configured to define a series of upwardly facing and sharpened edges. The wickers 19 bite into the outer surface of the fish downhole (not shown) in order to accomplish the retrieval function of the overshot tool 10. To this end, the grapple 18 is slotted, allowing it to collapse around the cylindrical item downhole.
Referring again to FIG. 1, the overshot tool 10 has an inner bore 15 that extends along its length. The inner bore 15 is dimensioned to receive, or “swallow,” the cylindrical body to be retrieved (not shown). The upper end of the cylindrical body to be retrieved tags the lower end of the top sub 11 as it is received within the overshot tool 10. To this end, the bottom end of the top sub 11 defines a reduced inner diameter portion that forms a shoulder 17. As the cylindrical body to be retrieved is received within the bore 15 of the overshot tool 10, the wickers 19 of the gripping member, or “grapple,” frictionally engage the outer surface of the cylinder being retrieved. At the same time, as the operator of the rig senses that the cylinder to be retrieved has tagged the sub 17, the operator begins to pull on the working string. As the operator pulls upward on the working string, the smooth outer surface of the grapple 18 is forced to ride downward along the ramped surfaces 16 of the body 12. This, in turn, causes the grapple 18 to more tightly engage the cylinder being retrieved.
The Bowen-type overshot tool, such as the one shown in FIG. 1, has provided a reliable means for retrieving parted pipe and other cylindrical bodies which have become lost downhole. This is at least true in the case of more shallow and generally vertical wells. However, in some instances, a string of pipe or tool simply cannot be retrieved. In this situation, the overshot tool 10 must be released from the cylindrical pipe segment downhole and then removed from the wellbore. To accomplish separation of the overshot tool 10 from the cylindrical body downhole, the operator of the rig applies a downward load on the overshot 10 and rotates the working string to the right, causing the gripping member 18 to unthread from the gripped cylindrical body downhole. To this end, the wickers 19 on the inner surface of the grapple 18 are cut in a spiraled arrangement to allow “unthreading.”
As noted, the Bowen overshot tool design has been a reliable standard for many years. It has proved successful in the more shallow wells and vertical wells historically drilled. However, during the last decade drilling activity (at least for the major U.S. oil companies) has shifted towards the drilling of deeper wells, and the drilling of lateral wells and extended reach wells. In these instances, the overshot tool 10 cannot be reliably released simply by “unthreading” the connection with the cylindrical body downhole. Those skilled in the art will understand that there is not a direct correlation between the rotation of the drill string at the surface of a well and rotation of the overshot tool downhole. This places the operator of a rig in a difficult dilemma when drilling a deep well or a well that is being drilled at a substantial angle of deviation. In this respect, the operator has two choices: (1) incur the expensive rig time needed in order to attempt to retrieve a downhole tool, such as an expensive drill bit carrying directional equipment, knowing that if the retrieval operation is unsuccessful the overshot tool will have to be left in the hole along with the expensive drilling or other equipment; or (2) avoid this risk and the expense of rig time and drill a new deviated hole in the wellbore at a measured depth above the point at which the drill pipe and connected tools have become lodged.
In an effort to make overshot tools more easily retrieved in the event of an unsuccessful retrieval operation, hydraulically released overshot tools have been developed. An example of such a hydraulically actuated overshot tool is found in U.S. Pat. No. 5,242,201 issued in 1993 to Beeman. The overshot tool in the '201 patent is hydraulically actuated for both catching and for releasing the fishing tool from the “fish.” Another example of an overshot tool is seen in U.S. Pat. No. 5,580,114 issued in 1996 to Palmer. The '114 patent represents another hydraulically actuated overshot tool. In the tool of the '114 patent, the cylindrical fish sought to be retrieved is mechanically caught, and hydraulically released.
In both the '201 Beeman fishing tool and the '114 Palmer fishing tool, a nozzle is placed within the inner bore of the tool. Without describing details of operation of the respective tools, each tool each utilizes a nozzle in order to create a pressure differential above and below the outlet of the nozzle. In the '201 Beeman patent, the nozzle is identified as a tapered segment 56 of a mandrel 10. In the '114 Palmer patent, the nozzle is a collet body, identified as item 28 in FIG. 1, and item 58 in FIG. 5. The pressure drop is created through the injection of fluid into the working string under pressure. The pressure differential acts upon the nozzle, causing the nozzle to act as a piston member.
The use of hydraulically actuated overshot tools has the advantage of avoiding the necessity of turning the drill string to release the cylindrical body attempting to be retrieved. At the same time, the presence of a nozzle in the overshot tools of the prior art presents several disadvantages. First, the nozzle creates a restriction within the bore for running additional tools downhole. For example, it is sometimes desirable to deploy a shot charge downhole on a wire line. The shot charge is used to create acoustic energy in order to separate joints of pipe for retrieval downhole. Second, the nozzle is sometimes asked to serve a stop function for which it was not designed. In this regard, the top end of the cylindrical body being retrieved most commonly tags the nozzle as the overshot tool is being lowered downhole. This, in turn, jars the nozzle upward relative to the housing of the tool. Again, without discussing details of the overshot tool, this may end up canceling out the piston function of the overshot tool, causing the overshot tool to be irretrievably engaged to the cylindrical body downhole. If this occurs, the overshot tool cannot hydraulically release.
Another disadvantage to the use of hydraulically actuated overshot tools relates to the placement of the proper pressure differential above the nozzle. In this respect, the item being retrieved from the well is commonly plugged or severely restricted. An example is where a mud motor is lodged at the lower end of a wellbore. This situation prevents pumping at a high enough flow rate to generate the pressure drop needed across the nozzle in order to actuate the tool. Still further, the inner diameter of the nozzle in a hydraulically actuated overshot tool prevents the use of extensions in the overshot. Sometimes, particularly when retrieving a mud motor, the overshot tool must be configured to “swallow” the shaft in order to frictionally engage the housing of the mud motor.
It can thus be seen then that a need exists for an improved overshot tool that employs the gripping capability of the Bowen-type overshot tool, but does not require turning of the drill string in order to effectuate a release of the overshot tool from the item being retrieved. Stated another way, a need exists for an overshot tool having the benefits of the Bowen-type fishing tool, but that does not require “unthreading” of the tool from the item being retrieved in the event retrieval is not successful. Further, a need exists for an overshot tool that can be hydraulically released from an item being retrieved, but which does not employ a nozzle within the bore of the overshot tool. Still further, a need exists for an overshot tool which can be hydraulically released from an item attempting to be retrieved from a wellbore, but which accommodates extensions so as to “swallow” elongated portions of the item being retrieved.